Actually… I think blog directory submissions are a waste of time.
As long as you have your RPC pingomatic ( http://codex.wordpress.org/Update_Services ) in place, your theme is validating ( http://http://validator.w3.org/ ) properly AND you are writing useful relevant unique content — the big blog search engines and web search engines WILL find you and index you.
One thing to do to have more exposure, is to be an active commentator on your subject in forums, blogs and discussions, linking yourself to your relevant content. Those links accumulate.
Read also tips on writing viral content http://www.google.com/search?q=viral+content .
If this suggestion helps, I invite you to my profile page.
